1. Field of the Invention
The present invention relates to flue gas desulfurization (FGD) systems generally and more particularly to wet FGD systems having spray headers and tray zones for desulfurization slurry liquor contact with flue gases.
2. Description of the Related Art
Prior art flue gas desulfurization (FGD) units are either wet or dry scrubber systems which use either solid or liquid desulfurization materials in contact with the flue gas.
A popular wet scrubber design today is the spray tower 10 depicted in FIG. 1. The tower 10 is designed so that, at maximum load, the average superficial gas velocity does not exceed the design gas velocity. For most spray towers, the average gas velocity varies from about 8 to 13 ft/s (2.4 to 4 m/s) based upon scrubber outlet conditions. A typical design velocity for a limestone wet scrubber is about 10 ft/s (3.1 m/s).
The flue gas enters the tower 10 from the side and gas flow nonuniformity is a potential problem. This problem may be eliminated with central or annual gas inlet designs newly implemented by The Babcock & Wilcox Company.
The absorber design depicted in FIG. 1 incorporates a prior art sieve or perforated plate tray assembly 12 which reduces flue gas flow maldistribution. The pressure drop across the tray is typically in the range of 1 to 3 in. H.sub.2 O (0.025 to 0.075 kPa). Towers with multiple trays of this design have also been built. The design of the tower and the number of trays is influenced by the reagent (lime or limestone, for example), the desired SO.sub.2 removal level, the trade-off between fan power and recirculation slurry pump power, and other process variables.
The dominant design variable for all FGD wet scrubbers is the ratio of slurry flow to gas flow in the tower, known as L/G. In the U.S., this term is expressed as gallons per thousand cubic feet of flue gas evaluated at scrubber outlet conditions. In Japan and Germany, the L/G units are l/m.sup.3. A wide range of L/G values have been used on wet scrubbers. Soda scrubbers operate at L/G ratios as low as 10 gal/1000 ft.sup.3 (1.34 l/m.sup.3) and some limestone scrubbers operate at L/G ratios up to 150 gal/1000 ft.sup.3 (20 l/m.sup.3).
Spray nozzles 14 are used in wet scrubbers to achieve a surface area contact between slurry with flue gas. The operating pressures typically vary between about 5 and 20 psi (34 and 138 kPa). Spray nozzles without internal obstructions are favored to minimize plugging by debris. Although plugging could be minimized by using a minimum number of large capacity spray nozzles, flow maldistribution would most likely occur. Therefore, a larger quantity of smaller capacity nozzles are usually preferred. Due to the abrasive nature of the slurry solids, the nozzles are manufactured out of ceramic or polymeric material to resist erosion.
The large tank at the bottom of the tower is typically referred to as the reaction tank or the recirculation tank. The volume of this tank permits several chemical and physical processes to approach completion. Some of these time dependant processes will be described in more detail later.
Flue gas enters the side of the scrubber module at a temperature typically within the range of 250.degree. to 350.degree. F. (121.degree. to 177.degree. C.) and is evaporatively cooled to its adiabatic saturation temperature by contact with the slurry. The scrubber inlet must be designed to prevent deposition of slurry solids at the wet-dry interface. Because the inlet flue is at the flue gas temperature [for example 300.degree. F. (149.degree. C.)] and the shell of the scrubber is at the saturation temperature [for example 125.degree. F. (52.degree. C.)], there exists a region where the surface temperature abruptly changes. Deposits are most likely to form in this region. Deposition is minimized by a combination of features which prevent periodic slurry splashing on the hot, dry side of the wet-dry line.
Flue gas passes vertically upward through the scrubber. In the unit illustrated in FIG. 1, the gas flow is uniformly distributed by a perforated plate or sieve tray 16. This tray also serves as a gas-liquid contacting device. Gas-liquid contact is enhanced by a froth of slurry which forms on the tray as the flue gas rises upward.
Above the tray, the flue gas passes through several spray levels to achieve additional gas-liquid contact. Each spray level consists of an array of headers and spray nozzles as shown in FIG. 1. The spray nozzles produce a relatively coarse spray. Typically the suspension of spray droplets is in countercurrent contact with the flue gas for about one to three seconds. A majority of the SO.sub.2 absorption occurs during this short contact time. The spray zone in combination with the slurry froth on the tray is referred to as the gas-liquid contact zone of the wet scrubber.
A disengagement height is provided above the top spray zone before the flue gas reaches the mist eliminators. The purpose is to allow disengagement and return via gravitational forces of the largest slurry droplets to the spray zone. For a scrubber operating at an average gas velocity of 10 ft/s (3 m/s), droplets larger than about 600 microns may have sufficient mass to fall back to the spray zone.
Typically chevron type mist eliminators are used in wet scrubbers to capture spray droplets entrained in the flue gas (see FIG. 3). These mist eliminators typically collect slurry deposits by impaction. They efficiently collect droplets larger than about 20 microns in diameter.
The design of the flue from the exit of the wet scrubber to the stack is an important facet of the system design. The potential for severe corrosion and deposition in these flues is well documented. This potential for severe corrosion arises from a combination of facts. First, the flue gas leaves the mist eliminator saturated with water vapor. Second, some carryover of slurry droplets smaller than 20 microns is inevitable. These droplets will usually be slightly acidic and may contain high concentrations of dissolved chlorides. The flue gases will contain some residual SO.sub.2 and ample oxygen to oxidize some of the SO.sub.2 to SO.sub.3. Because the flue gas is saturated with water vapor, surface condensation is inevitable. This condensate can become severely acidic (pH less than 1.0), and calcium salts can deposit on the walls. Two approaches are used to minimize these effects, flue gas reheat and flue/stack lining. The former option involves reheating the flue gas to vaporize entrained droplets. There are various methods for reheating the flue gas that exits the scrubber. Some examples include the following:
1. steam coil heaters; PA0 2. mixing with some hot flue gas which is bypassed around the scrubber; PA0 3. mixing with hot air; PA0 4. mixing with hot gases generated by combustion of a clean fuel; and PA0 5. regenerative heat exchanger which transfers heat from the hot flue gas inlet to the cooler flue gas outlet.
Several problems are associated with each of the reheat methods. Deposition and corrosion occur in the heat exchanger. Reheating with bypass gas reduces the overall FGD system effectiveness. Finally, the evaporation of droplets from the scrubber concentrates the corrosive constituents in the slurry. As a result, operation with out flue gas reheat, (i.e., with a wet stack), has become popular in the U.S. Under these conditions, the flue from the scrubber to the stack is lined with corrosion resistant materials, and the stack is lined with acid resistant brick or other suitable material. A drainage system is also included to accommodate condensation of water vapor.